Magnetic structure of the kagome lattice antiferromagnet potassium jarosite KFe3(OH)6(SO4)2

We have examined the magnetic structure of the kagome lattice antiferromagnet potassium jarosite (K jarosite: KFe3(OH)(6)(SO4)(2)) by means of powder neutron diffraction. Extremely high degeneracy of the ground states prevents the long-range magnetic ordering at any temperature and the root 3x root 3 structure is predicted theoretically to be favored rather than the q=0 structure at T=0 in a kagome lattice Heisenberg antiferromagnet. Nevertheless, K jarosite shows long-range magnetic ordering at 65 K and the ordered magnetic structure was found to be the q=0 structure. In addition, although the q=0 structure has two degenerated states of "positive" and "negative" chirality, the observed magnetic structure contains only elemental triangles of positive chirality. We found that a weak single-ion-type anisotropy is crucial for selecting the observed magnetic structure. The long-range magnetic ordering at finite temperature in the jarosite family of compounds can be ascribed to this anisotropy.